A projection exposure apparatus has been conventionally used to project a circuit pattern of a reticle (or a mask) via a projection optical system onto a wafer etc, and high-resolution and high-throughput exposures have recently been increasingly demanded. The immersion exposure has attracted people's attentions as one means that meets the high-resolution demand. The immersion exposure promotes a higher numerical aperture (“NA”) of the projection optical system by replacing a medium at the wafer side of the projection optical system with a liquid. The projection optical system has an NA=n· sin θ where n is a refractive index of the medium, and the NA increases up to n when the medium that has a refractive index higher than that of air, i.e., n>1. As a result, the resolution R (R=k1(λ/NA)) of the exposure apparatus expressed by a process constant k1 and a light source wavelength λ becomes small.
A local fill that locally fills a liquid in a space between the projection optical system and the wafer is proposed in the immersion exposure. See, for example, International Publications Nos. 99/49504 and 2004/086470. An gas curtain system is also proposed which blows a gas against the interface of the liquid that has been supplied between the projection optical system and the wafer, and limits the liquid between them. See, for example, Japanese Patent Application, Publication No. 2004-289126.
However, in Japanese Patent Application No. 2004-289126, the gas pressure of the gas curtain that limits the liquid between the projection optical system and the wafer so weak that the filled liquid disperses as the wafer moves at a high speed during exposure.
Insufficient filling of the liquid would cause a mixture of gas bubbles in the liquid. The gas bubbles mixed in the liquid diffusely reflect the exposure light, and reduce the exposure dose and thus the throughput. When the gas curtain around the liquid has a low humidity, the liquid evaporates and causes the evaporation heat. Due to this evaporation heat, not only the liquid cools but also the projection optical system and wafer that contact the liquid cool, causing their deformations and lowered exposure accuracy. In addition, it is not always economical to create the gas curtain throughout the contour of the liquid.